The blue and white selectable markers differentiate recombinant colonies from non-recombinant colonies on the basis of their abil…

Question

The blue and white selectable markers differentiate recombinant colonies from non-recombinant colonies on the basis of their ability to produce colour in the presence of a chromogenic substrate. Given below are two statements:

Statement I: The blue coloured colonies have DNA insert in the plasmid and they are identified as recombinant colonies.

Statement II: The colonies without blue colour have DNA insert in the plasmid and are identified as recombinant colonies. In the light of the above statements, choose the most appropriate answer from the options given below:

1. A. Both Statement I and Statement II are correct
2. B. Both Statement I and Statement II are incorrect
3. C. Statement I is correct but Statement II is incorrect
4. D. Statement I is incorrect but Statement II is correct (Correct answer)

Correct Answer

Option D — Statement I is incorrect but Statement II is correct

Detailed Solution & Explanation

The correct answer is Statement I is incorrect but Statement II is correct.

Key Points

• The DNA insert goes into the coding sequence of $\beta$-galactosidase, causing insertional inactivation of that gene.
• Recombinants (with insert) cannot make the enzyme, so they produce no colour (white colonies) — Statement II is correct.
• Non-recombinants have an intact $\beta$-galactosidase gene and produce blue colonies — so Statement I (blue = recombinant) is incorrect.